Use of engineered unique cysteine residues to facilitate oriented coupling of proteins directly to a gold substrate.

A prerequisite for any "lab on a chip" device that utilizes an electrical signal from the sensor protein is the ability to attach the protein in a specific orientation onto a conducting substrate. Here, we demonstrate the covalent attachment to a gold surface of light-harvesting membrane proteins, from Rhodobacter sphaeroides, via cysteine (Cys) residues engineered on either the cytoplasmic or periplasmic face. This simple directed attachment is superior in its ability to retain light-harvesting complex (LHC) function, when compared to a similar attachment procedure utilizing a self-assembled monolayer on gold. LH 1 has previously been observed to have superior photostability over LH 2 (Magis et al. [2010] Biochim. Biophys. Acta, 1798, 637-645); this characteristic is maintained even with the introduction of Cys residues.

Light harvesting, energy transfer and electron cycling of a native photosynthetic membrane adsorbed onto a gold surface.

Photosynthetic membranes comprise a network of light harvesting and reaction center pigment-protein complexes responsible for the primary photoconversion reactions: light absorption, energy transfer and electron cycling. The structural organization of membranes of the purple bacterial species Rb. sphaeroides has been elucidated in most detail by means of polarized light spectroscopy and atomic force microscopy. Here we report a functional characterization of native and untreated membranes of the same species adsorbed onto a gold surface. Employing fluorescence confocal spectroscopy and light-induced electrochemistry we show that adsorbed membranes maintain their energy and electron transferring functionality. Gold-adsorbed membranes are shown to generate a steady high photocurrent of 10 microA/cm(2) for several minutes and to maintain activity for up to three days while continuously illuminated. The surface-adsorbed membranes exhibit a remarkable functionality under aerobic conditions, even when exposed to light intensities well above that of direct solar irradiation. The component at the interface of light harvesting and electron cycling, the LH1 complex, displays exceptional stability, likely contributing to the robustness of the membranes. Peripheral light harvesting LH2 complexes show a light intensity dependent decoupling from photoconversion. LH2 can act as a reversible switch at low-light, an increased emitter at medium light and photobleaches at high light.

Probing the electronic structure and conformational flexibility of individual light-harvesting 3 complexes by optical single-molecule spectroscopy.

We present fluorescence-excitation spectra of individual light-harvesting 3 (LH3 or B800-820) complexes of Rhodopseudomonas acidophila at 1.2 K. The optical single-molecule studies were employed to investigate the electronic structure as well as the conformational flexibility of the individual pigment-protein complexes. The optical spectra resemble those of individual light-harvesting 2 (LH2) complexes, in agreement with the structural similarity of both types of complexes. Although variations among the LH3 spectra are large, there is a distinct difference in the spectral features of the 800 and 820 nm region that appears in all the complexes studied. In the B800 region 4-6 narrow bands are present whereas in the B820 region a limited number of relatively broad bands are observed. These observations can generally be interpreted in terms of localized excitations in the 800 nm region and delocalized excitations in the 820 nm region. The observed heterogeneous spectral behavior, especially in the B820 band, indicates that the B820 pigments of LH3 are sensitive to light-induced local conformational changes. It is suggested that a rotation of the C(3)-acetyl chain of a BChl a pigment bound to the beta-subunit of the light-harvesting complex is the origin of the conformational flexibility and affects the optical properties of the whole pigment-protein complex.